The cryogenic separation of air requires a pre-purification step for the removal of both high-boiling and hazardous materials. Principal high-boiling air components include water (H2O) and carbon dioxide (CO2). If removal of these impurities from an ambient feed air is not achieved, then H2O and CO2 will freeze out in cold sections of the separation process, such as heat exchangers and the liquid oxygen (LOX) sump. This can cause pressure drop, flow variations, and also lead to operational problems. In addition, the high boiling hydrocarbons, if not removed, will concentrate in the LOX section of the column to produce flammable mixtures, resulting in a potential explosive hazard. It is also desired that various hazardous materials present in feed air including hydrocarbons such as ethylene, acetylene, butane, propylene and propane be removed prior to introduction to the air separation unit (ASU). Such materials can concentrate within the ASU and form flammable mixtures with oxygen or enriched air.
To avoid accumulation of these impurities in the plant, a certain portion of the liquid oxygen produced must be purged from the system to avoid concentration of these impurities. This purging of liquid oxygen reduces the overall recovery of the plant and lowers possible recovery of other high boiling components like argon, krypton and xenon.
It is also known that oxides of nitrogen should be removed prior to cryogenic separation. A minor air component is nitrous oxide (N2O), which is present in ambient air at about 0.3 ppm. It has similar physical properties to carbon dioxide and therefore presents a potential operation problem because of solids formation in the column and heat exchangers of the cryogenic distillation apparatus. In addition, N2O is known to enhance combustion of organic materials and is shock sensitive. The removal of N2O from air prior to cryogenic distillation therefore has a number of advantages. First, it improves the overall safety operation of the air separation unit (ASU). Second, it allows for reduced liquid oxygen purge which improves the plant recovery of oxygen and rare gases. Third, it allows for the use of downflow reboilers, which require high levels of N2O removal. Downflow reboilers, as opposed to thermosiphon reboilers, are more efficient and lower the overall power required for oxygen production. As such, nitrous oxide also presents a significant safety hazard and thus there is significant interest to remove trace N2O from air prior to cryogenic distillation.
The pre-purification of air is usually conducted by adsorptive clean-up processes in which contaminating gas components are adsorbed on solid adsorbents with periodic regeneration of the adsorbent. Such processes include pressure swing adsorption (PSA) (U.S. Pat. No. 5,232,474), temperature enhanced pressure swing adsorption (TEPSA) (U.S. Pat. No. 5,614,000), or temperature swing adsorption (TSA) (U.S. Pat. Nos. 4,541,851 and 5,137,548). There is no requirement for regeneration of heat energy in PSA processes as opposed to TEPSA or TSA processes.
When there is sufficient waste gas (purge gas) available in a cryogenic air separation plant, the PSA process is usually a preferred option for air pre-purification due to its simplicity, lower capital cost, and lower operating cost. PSA generally involves coordinated pressure cycling of a gaseous mixture over an adsorbent material. The total pressure is elevated during intervals of flow in a first direction through the adsorption bed, and is reduced during intervals of flow in the reverse direction, during which the adsorbent is regenerated. As the cycle is repeated, the less readily adsorbed component is concentrated in the first direction, while the more readily adsorbed component is concentrated in the reverse direction.
In general, these systems are designed for total H2O and CO2 removal from ambient air. Typically, these systems are run until CO2 reaches a certain low level of breakthrough (time average 10-100 ppb). So, in most plants, measurement of CO2 breakthrough level is used to ensure reliable operation of the plant (i.e. no operational problems).
U.S. Pat. No. 6,106,593 describes a TSA process that employs a three-layer system consisting of alumina (for H2O removal), 13X (for CO2 removal) and CaX (for N2O and hydrocarbon removal). The resulting N2O removal at 20-50 ppb CO2 time averaged in the air product is 93% as well as removing 100% of inlet ethylene. U.S. Pat. No. 8,734,571 described a PSA process for the removal of N2O from ambient air in which bed comprising alumina (85%) at the feed end of the bed and 13X zeolite (15%) at the product end of the bed remove only 83% of the inlet N2O at a CO2 breakthrough level of 50 ppb. It would be of interest to the industry to develop a PSA process where at CO2 breakthrough levels of 20-50 ppb, both N2O and hydrocarbons are removed.
The adsorbents in the PSA devices do not normally get completely regenerated at the completion of the purge step and hence their dynamic capacity, the ability to remove the desired components, is reduced compared to TEPSA or TSA processes. As a result, the PSA process is typically run for shorter cycle times than TSA or TEPSA thus the bed(s) undergo blow down and re-pressurization at fairly frequent intervals during which the feed gas is vented off. During the blowdown step, there is a noticeable loss of air trapped within the void spaces of the vessel(s) and piping as well as the air adsorbed on the adsorbents. This collective air loss, referred to by various terms such as blowdown loss, vent loss or “switch loss”, can represent a significant energy waste as the air is compressed but not utilized for air separation downstream of the pre-purifier. Reducing switch loss can provide significant operating cost savings in terms of reduced compression power.
In PSA processes it is usual to use two adsorbent beds, with one being on-line while the other is regenerated. The depressurization and regeneration of one bed must take place during the short time for which the other bed is on-line, and rapid repressurization can lead to transient variations in the feed and product flows which can adversely affect plant operation.
Much of the existing art focuses on reducing or minimizing the switch loss in a PSA pre-purification process. One method is the use of an adsorbent configuration with a larger proportion of a weak adsorbent such as activated alumina or its modified form which has very low capacity for O2 and N2, and a relatively smaller proportion of the stronger adsorbent such as a molecular sieve for optimized performance (U.S. Pat. Nos. 4,711,645; 5,769,928; 6,379,430 B1 and 5,656,064). Another approach is to reduce the frequency of the blowdown or bed switch loss mentioned above, by optimizing bed layering, using composite adsorbents or purging beds at slightly elevated temperature (U.S. Pat. No. 7,713,333, U.S. Pat. No. 5,855,650). These methods however do not try to reduce the amount of adsorbent used or the size of adsorber vessels. On the contrary, they often result in increased adsorbent inventory.
Conventional PSA pre-purifiers normally operate two adsorbers at cycle times in the order of minutes. For small to medium scale air separation plants, the sizes of the pre-purifier vessels are often too large to fit inside a container. This results in difficulty of shipping, relocating and high cost of installation. It is therefore desirable to reduce the overall footprint of the air pre-purification device.
Reduction in the size and the cost of PSA device, and increase in PSA productivity can be realized by process intensification. One of the common methods of PSA process intensification is to reduce the cycle time of the device. However, an inherent challenge of so-called rapid cycle pressure swing adsorption (RCPSA) systems is that as cycle time decreases, there is a need for faster mass transfer adsorbents.
A number of different adsorbents are known for use in PSA processes. For example, U.S. Pat. No. 5,779,767 describes the use of composite adsorbents comprising a mixture of alumina and zeolite in normal cycle PSA processes for the removal of CO2 and H2O from an ambient air stream. It has also been demonstrated that activated alumina powder and zeolite powder can be used to form composite adsorbent beads which can be used to purify an air stream in order to remove H2O, CO2 and other impurities including hydrocarbons (see U.S. Pat. Nos. 7,115,154 and 6,638,340).
Typically, improved mass transfer in solid adsorbents is achieved by reducing the particle size (see for example U.S. Pat. No. 5,232,474 and U.S. Pat. No. 8,192,526). The higher mass transfer rate shortens the mass transfer zone, and/or allows the PSA process to run at reduced cycle time. However, the decreased cycle time also results in higher gas velocities in the adsorber which in turn results in a higher pressure drop. The small particles and higher pressure drop will eventually lead to undesirable particle fluidization.
It is also known to use supported adsorbent materials (i.e. laminates) in PSA processes. For example, the use of structured laminate beds for RCPSA processes with ultra-short cycle times has also be been described for H2 purification and O2 vacuum swing adsorption (VSA) applications (U.S. Pat. Nos. 7,300,905, 7,037,358, 7,763,098 and U.S. Pat. No. 8,303,683 B2). In particular, U.S. Pat. No. 7,037,358 describes supported laminates having a guard layer and an adsorbent layer in RCPSA applications. The guard layer removes contaminants, particularly H2O from the feed stream before contacting the adsorbent layer.
The effect of cycle time on product purity has been also studied in the existing art. For example, Gomes et al have shown that in a PSA process for the separation of C02 and N2, as cycle time decreases, the purity of N2 decreases (Separation and Purification Technology, 2002, 28, 2, 161-171). In addition, Farooq et al have investigated the production of O2 from ambient air using PSA both experimentally and with simulations (Chemical Engineering Science, 1989, 44, 12, 2809). The experimental data shows that O2 purity actually goes through a maximum as cycle time decreases and the simulations show that the purity decreases at shorter cycle time.
There remains a need to provide an improved process for the removal of impurities from a feed air stream.